When semiconductor wafers are processed, they typically undergo a number of photolithographic processes in which photoresist is applied, exposed and developed over the wafer. Photoresist is utilized to define patterns of circuit features which can then be etched or diffused into the substrate or various layers formed thereover. It is highly desirable to provide a photoresist layer which is uniform in its thickness across the wafer.
A typical photolithographic process can involve the following steps. A wafer is typically pretreated in order to obtain smooth, uniform coverage of the photoresist with good adhesion of the resist to the wafer. Such pretreatment can involve a dehydration bake which is intended to drive off most of the water on the surface of the wafer. The wafer can then be treated with a chemical primer to promote adhesion of the photoresist. After priming, the wafer can be coated with photoresist. Most commonly, photoresist is applied by spin-coating it onto the wafer. After spin-coating, the wafers typically undergo baking which drives off solvent in the photoresist. After the baking, the wafer can be exposed and developed.
Situations can arise during photolithographic processing, and in particular with the after-coating baking, which can cause the resultant photoresist to have a characteristic slope in its thickness across the wafer. One cause of this characteristic slope is due to air flow inside, or around various apparatus which are used to bake the photoresist. Specifically, air flow inside or around these apparatus can cause one side of a wafer to bake or cool differently than another side of the wafer. This variability can cause the characteristic slope in the resist thickness. Additionally, some processing can involve inserting coated wafers into a processing chamber containing a hot plate for heating wafers. The wafers are typically inserted from one end of the chamber. Conditions within the chamber can cause the side of the wafer closest to the chamber end from which it was inserted to be cooler than other portions of the wafer disposed further within the chamber. Such can lead to thickness uniformity problems, such as producing a thicker resist layer towards the insert end of the chamber versus the fixed end wall portion of the chamber. Hence, due to temperature irregularities and other effects, a thickness gradient can occur resulting in thicker photoresist on one side of the wafer at the conclusion of heat processing within the chamber.
Accordingly, this invention arose out of concerns associated with providing improved methods and apparatus for forming uniformly-thick photoresist layers.